Usually, when converting an older existing moored drilling rig to a dynamically positioned (DP) drilling vessel, a number of thrusters are added to the pontoon bodies and a large amount of power generation and distribution equipment is necessarily added to the work deck. This equipment includes, among other things, diesel generators, switchboards, transformers, variable speed drives for thrusters, support equipment for the diesel generators and control equipment for keeping the rig on station.
Additionally, the drilling deck's capability for carrying load has to be increased to accommodate the DP support equipment and the possibility of additional drilling equipment. Usually the increased weight and payload are compensated for by adding sponsons to the submerged pontoon bodies and by adding fenders to the support columns. Both measures enhance the carrying and stabilizing capacity of the vessel. Also, the deck structure and the bracing system usually will have to be upgraded. All these structural alterations lead to extensive and expensive welding labor.
The integration of new thrusters into the pontoon bodies and the installation of new power generation and distribution equipment into the existing pontoon and deck layout (while at the same time meeting separation and redundancy requirements for a dynamically positioned rig) involves substantial time, complex routing and installation of piping and cables. The time schedule for performing such a conversion normally means that the existing rig must be taken out of operation for a long time period for the necessary construction to be completed.
In order for a conversion to be cost-effective, one usually desires to start with an existing, but comparatively modern drilling rig having a large deck-load capacity. Such rigs, however, are usually profitable in their normal role and original configuration and therefor are seldom cheaply available for conversion.
Other types of semi-submersible conversions are known; for example, the owner of the present invention is also the owner of the invention disclosed in U.S. Pat. No. 5,036,782 entitled Method For Converting a Semi-Submersible Vessel. The '782 patent, however, discloses an enhancement to an existing semi-submersible drilling- or accommodation-rig for accomplishing a conversion into a floating production plant having apparatus for extracting and processing oil and/or gas, or other heavy and bulky deck loads. The disclosed apparatus and method for the conversion taught by the '782 patent, however, is distinctly different from the apparatus and method disclosed herein. Importantly, the conversion taught by the '782 patent does not enable dynamic station keeping and/or positioning.